Precision spectral filters filter specific wavelengths of visible or UV radiation. This allows for the production of optical articles, such as glasses, which can be tailored to block specific wavelengths of light to produce optical articles for different uses, including the protection of the cornea, lens and retina from specific harmful radiation wavelengths. For example, various sunglasses have been used to protect human eyes from strong light including photochromic glasses, polarized glasses and glasses for specific activities including shooting and fishing. Photochromic spectacles darken upon exposure to certain wavelengths of light and typically exposure to ultraviolet (UV) light and brighten when UV light is removed. Often, such photochromic spectacles include a prescription for vision correction.
Adapting certain technology, including photochromic technology to contact lenses is more difficult than adapting the same technology to spectacles. Additional factors, such an oxygen permeability, comfort and fit of the resulting lens, must be taken into account. The manufacturing process of contact lenses is also more complicated. Typically, contact lenses are formed by irradiating a photoinitiator in the presence of one or more polymerizable materials. In the case of photochromic contact lenses, it is desirable to include the photochromic dye in the reactive mixture containing the photoinitiator and polymerizable materials that, upon polymerization, forms the contact lens. Unfortunately, certain dyes, including photochromic dyes have the potential to interfere with the activation of the photoinitiator.
Polymerizable mixtures may also be cured using other free radical based chain reaction polymerization, including thermal polymerization.